8/22/2019 Chapter5- Forces in Equilibrium http://slidepdf.com/reader/full/chapter5-forces-in-equilibrium 1/24 109 Chapter 5 Forces in Equilibrium Many people would not consider it extraordinary to get into an elevator and zoom to the top of a 50 story building. They might not be so nonchalant if they knew the balance of enormous forces that keeps a tall building standing up. Or, they might feel even more secure, knowing how well the building has been engineered to withstand the forces. Tall buildings are an impressive example of equilibrium, or the balancing of forces. The average acceleration of a building should be zero! That means all forces acting on the building must add up to zero, including gravity, wind, and the movement of people and vehicles. A modern office tower is constructed of steel and concrete beams that are carefully designed to provide reactions forces to balance against wind, gravity, people, and vehicles. In ancient times people learned about equilibrium through trial-and-error. Then, as today, different builders and architects each wanted to make a building taller than the others. Without today's knowledge of equilibrium and forces, many builders experimented with designs that quickly fell down. It is estimated that ten cathedrals fell down for every one that is still standing today! Over time, humans learned the laws of forces and equilibrium that allow us to be much more confident about the structural strength of modern tall buildings. Key Questions 3 How do you precisely describe a force? 3 How is the concept of equilibrium important to the design of buildings and bridges? 3 What is friction? 3 How is torque different from force?
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Think about how to accurately describe a force. One important piece of information is the strengthof the force. For example, 50 newtons would be a clear description of the strength of a force. But
what about the direction? The direction of a force is important, too. How do you describe the
direction of a force in a way that is precise enough to use for physics? In this section you will learn
that force is a vector . A vector is a quantity that includes information about both size (strength) and
direction.
Scalars and vectors
Scalars have
magnitude
A scalar is a quantity that can be completely described by a single value
called magnitude. Magnitude means the size or amount and always includes
units of measurement. Temperature is a good example of a scalar quantity. If
you are sick and use a thermometer to measure your temperature, it might
show 101°F. The magnitude of your temperature is 101, and degrees
Fahrenheit is the unit of measurement. The value of 101°F is a complete
description of the temperature because you do not need any more
information.
Examples of
scalars
Many other measurements are expressed as scalar quantities. Distance, time,
and speed are all scalars because all three can be completely described with a
single number and a unit.
Vectors have
direction
Sometimes a single number does not include enough information to describe
a measurement. In giving someone directions to your house, you could not
tell him simply to start at his house and drive four kilometers. A single
distance measurement is not enough to describe the path the person mustfollow. Giving complete directions would mean including instructions to go
two kilometers to the north, turn right, then go two kilometers to the east
(Figure 5.1). The information “two kilometers to the north” is an example of
a vector . A vector is a quantity that includes both magnitude and direction.
Other examples of vectors are force, velocity, and acceleration. Direction is
important to fully describe each of these quantities.
Figure 5.1: Vectors are useful in giving
directions.
Vocabulary
scalar, magnitude, vector,component, free body diagram
Objectives
3 Draw vectors to scale torepresent a quantity’smagnitude and direction.
What is a forcevector? A force vector has units of newtons, just like all forces. In addition, the force
vector also includes enough information to tell the direction of the force.
There are three ways commonly used to represent both the strength and
direction information: a graph, an x-y pair, and a magnitude-angle pair. You
will learn all three in this chapter because each is useful in a different way.
Drawing a force
vector
The graph form of the force vector is a picture showing the strength and
direction of a force. It is just like an ordinary graph except the x- and y-axes
show the strength of the force in the x and y directions. The force vector is
drawn as an arrow. The length of the arrow shows the magnitude of the vector,and the arrow points in the direction of the vector.
Scale When drawing a vector, you must choose a scale. A scale for a vector diagram
is similar to a scale on any graph. For example, if you are drawing a vector
showing a force of five newtons pointing straight up (y-direction) you might
use a scale of one centimeter to one newton. You would draw the arrow five
centimeters long pointing along the y-direction on your paper (Figure 5.2).
You should always state the scale you use when drawing vectors. x and y forces When you draw a force vector on a graph, distance along the x- or y-axes
represents the strength of the force in the x- and y-directions. A force at an
angle has the same effect as two smaller forces aligned with the x- and y-
directions. As shown in Figure 5.3, the 8.6-newton and 5-newton forces
applied together have the exact same effect as a single 10-newton force
applied at 30 degrees. This idea of breaking one force down into an equivalent
pair of x- and y-forces is very important, as you will see.
An example Suppose three people are trying to keep an injured polar bear in one place.Each person has a long rope attached to the bear. Two people pull on the bear
with forces of 100 N each (Figure 5.8). What force must the third person apply
to balance the other two? The bear will not move if the net force is zero. To
find the answer, we need to find the net force when the forces are not in the
same direction. Mathematically speaking, we need a way toadd vectors.
Graphically
adding vectors
On a graph you add vectors by drawing them end-to-end on a single sheet. The
beginning of one vector starts at the end of the previous one. The total of all
the vectors is called the resultant. The resultant starts at the origin and ends atthe end of the last vector in the chain (Figure 5.9). The resultant in the example
is a single 141 newton force at 225 degrees. To cancel this force, the third
person must pull with an equal 141 N force in the opposite direction (45°).
Adding force vectors this way is tedious because you must carefully draw each
one to scale and at the proper angle.
Adding x-y
components
Adding vectors in x-y components is much easier. The x-component of the
resultant is the sum of the x-components of each individual vector. The y-component of the resultant is the sum of the y-components of each individual
vector. For the example, (-100, 0) N + (0, -100) N = (-100, -100) N. The
components are negative because the forces point in the negative-x and
negative-y directions. The resultant vector is (-100, -100) N.
Equilibrium To have zero net force, the forces in both the x and y directions must be zero.
The third force must have x and y components that add up to zero when
combined with the other forces. The solution to the problem is written below.
Following the rules we just gave, the third force must be (100, 100) N. This is
Finding the netforce For an object to be in equilibrium, all the forces acting on the object musttotal to zero. In many problems you will need the third law to find reaction
forces (such as normal forces) that act on an object.
Using vectors In equilibrium, the net force in each direction must be zero. That means the
total force in the x-direction must be zero and total force in the y-direction
also must be zero. You cannot mix x- and y-components when adding forces.
Getting the forces in each direction to cancel separately is easiest to do when
all forces are expressed in x-y components. Note: In three dimensions, there
also will be a z -component force.
Balancing forces If you are trying to find an unknown force on an object in equilibrium, the
first step is always to draw a free-body diagram. Then use the fact that the net
force is zero to find the unknown force. To be in equilibrium, forces must
balance both horizontally and vertically. Forces to the right must balance
forces to the left, and upward forces must balance downward forces.
Equilibrium
Two chains are used to lift a small boat weighing 1500 newtons. As the boat moves upward at a constant speed, one chain pullsup on the boat with a force of 600 newtons. What is the force exerted by the other chain?
1. Looking for: You are asked for an unknown force exerted by a chain.
2. Given: You are given the boat’s weight in newtons and the
force of one chain in newtons.
3. Relationships: The net force on the boat is zero.
4. Solution: Draw a free-body diagram (Figure 5.10).The force of the two chains must balance the boat’s weight.
600 N + F chain2 = 1500 N F chain2 = 900 N
Your turn...
a. A heavy box weighing 1000 newtons sits on the floor. You lift upward on the box with a force of 450 newtons, but the box
does not move. What is the normal force on the box while you are lifting? Answer: 550 newtons
b. A 40-newton cat stands on a chair. If the normal force on each of the cat’s back feet is 12 newtons, what is the normal force
on each front foot? (You can assume it is the same on each.) Answer: 8 newtons
Hooke’s law The relationship between a spring’s change in length and the force it exerts iscalled Hooke’s law. The law states that the force exerted by a spring is
proportional to its change in length. For example, suppose a spring exerts a
force of five newtons when it is stretched two centimeters. That spring will
exert a force of 10 newtons when it is stretched four centimeters. Doubling
the stretching distance doubles the force.
Spring constant Some springs exert small forces and are easy to stretch. Other springs exert
strong forces and are hard to stretch. The relationship between the force
exerted by a spring and its change in length is called itsspring constant. Alarge spring constant means the spring is hard to stretch or compress and
exerts strong forces when its length changes. A spring with a small spring
constant is easy to stretch or compress and exerts weak forces. The springs in
automobile shock absorbers are stiff because they have a large spring
constant. A retractable pen’s spring has a small spring constant.
How scales work The relationship between force and change in length is used in scales
(Figure 5.13). When a hanging scale weighs an object, the distance the springstretches is proportional to the object’s weight. An object that is twice as
heavy changes the spring’s length twice as much. The scale is calibrated
using an object of a known weight. The force amounts are then marked on the
scale at different distances. A bathroom scale works similarly but uses a
spring in compression. The greater the person’s weight, the more the spring
compresses.
5.2 Section Review
1. Can a moving object be in equilibrium? Explain.
2. Draw a free-body diagram of a 700-newton person sitting on a chair in equilibrium.
3. The spring in a scale stretches 1 centimeter when a 5-newton object hangs from it. How much does an object weigh if it stretches the spring 2 centimeters?
4. How is normal force similar to the force of a spring?
Friction forces are constantly acting on you and the objects around you. When you are riding a bicycle and just coasting along, friction is what finally slows you down. But did you know that
friction also helps you to speed up? Tires need friction to push against the road and create the
reaction forces that move you forward. In this section you will learn about different types of
friction, the cause of friction, and how it affects the motion of objects. You will also find out how
friction is useful to us and learn how to reduce it when it’s not.
What is friction?
What is friction? Friction is a force that resists the motion of objects or surfaces. You feel the
effects of friction when you swim, ride in a car, walk, and even when you sit in
a chair. Because friction exists in many different situations, it is classified into
several types (Figure 5.14). This section will focus on sliding friction and
static friction. Sliding friction is present when two objects or surfaces slide
across each other. Static friction exists when forces are acting to cause an
object to move but friction is keeping the object from moving.
The cause of friction
If you looked at a piece of wood, plastic, or paper through a powerful microscope, you
would see microscopic hills and valleys on
the surface. As surfaces slide (or try to slide)
across each other, the hills and valleys grind
against each other and cause friction.
Contact between the surfaces can cause the
tiny bumps to change shape or wear away. If
you rub sandpaper on a piece of wood, friction affects the wood’s surface andmakes it either smoother (bumps wear away) or rougher (they change shape).
Two surfaces are
involved
Friction depends on both of the surfaces in contact. The force of friction on a
rubber hockey puck is very small when it is sliding on ice. But the same
hockey puck sliding on a piece of sandpaper feels a large friction force. When
the hockey puck slides on ice, a thin layer of water between the rubber and the
ice allows the puck to slide easily. Water and other liquids such as oil can
greatly reduce the friction between surfaces. Figure 5.14: There are many types of friction.
Figure 5.15: The direction of friction isopposite the direction the box is pushed.
Figure 5.16: How the friction forces on the
box change with the applied force.
Identifying friction forces
Direction of thefriction force Friction is a force, measured in newtons just like any other force. You drawthe force of friction as another arrow on a free-body diagram. To figure out
the direction of friction, always remember that friction is a resistive force.
The force of friction acting on a surface always points opposite the direction
of motion of that surface. Imagine pushing a heavy box across the floor
(Figure 5.15). If you push to the right, the sliding friction acts to the left on
the surface of the box touching the floor. If you push the box to the left, the
force of sliding friction acts to the right. This is what we mean by saying
friction resists motion.Static friction Static friction acts to keep an object at rest from starting to move. Think
about trying to push a heavy box with too small a force. The box stays at rest,
therefore the net force is zero. That means the force of static friction is equal
and opposite to the force you apply. As you increase the strength of your
push, the static friction also increases, so the box stays at rest. Eventually
your force becomes stronger than the maximum possible static friction force
and the box starts to move (Figure 5.16). The force of static friction is equal
and opposite your applied force up to a limit. The limit depends on detailssuch as the types of surface and the forces between them.
Sliding friction Sliding friction is a force that resists the motion of an object already moving.
If you were to stop pushing a moving box, sliding friction would slow the box
to a stop. To keep a box moving at constant speed you must push with a force
equal to the force of sliding friction. This is because motion at constant speed
means zero acceleration and therefore zero net force. Pushing a box across
the floor at constant speed is actually another example of equilibrium. In thiscase the equilibrium is created because the force you apply cancels with the
force of sliding friction.
Comparing static
and sliding
friction
How does sliding friction compare with the static friction? If you have ever
tried to move a heavy sofa or refrigerator, you probably know the answer. It is
harder to get something moving than it is to keep it moving. The reason is
that static friction is greater than sliding friction for almost all combinations
The amount of friction that exists when a box is pushed across a smooth floor is greatly different from when it is pushed across a carpeted floor. Every
combination of surfaces produces a unique amount of friction depending upon
types of materials, degrees of roughness, presence of dirt or oil, and other
factors. Even the friction between two identical surfaces changes as the
surfaces are polished by sliding across each other. No one model or formula
can accurately describe the many processes that create friction. Even so, some
simple approximations are useful.
An example Suppose you pull a piece of paper across a table. To pull the paper at a constantspeed, the force you apply must be equal in strength to the sliding friction. It is
easy to pull the paper across the top of the table because the friction force is so
small; the paper slides smoothly. Do you believe the friction force between the
paper and the table is a value that cannot be changed? How might you test this
question?
Friction and the
force between
surfaces
Suppose you place a brick on the piece of paper (Figure 5.17). The paper
becomes much harder to slide. You must exert a greater force to keep the paper
moving. The two surfaces in contact are still the paper and the tabletop, so
why does the brick have an effect? The brick causes the paper to press harder
into the table’s surface. The tiny hills and valleys in the paper and in the
tabletop are pressed together with a much greater force, so the friction
increases.
The greater the force squeezing two surfaces together,
the greater the friction force.The friction force between two surfaces is approximately proportional to the
force the surfaces exert on each other. The greater the force squeezing the two
surfaces together, the greater the friction force. This is why it is hard to slide a
heavy box across a floor. The force between the bottom of the box and the
floor is the weight of the box. Therefore, the force of friction is also
proportional to the weight of the box. If the weight doubles, the force of
friction also doubles. Friction is present between all sliding surfaces.
Figure 5.17: Friction increases greatlywhen a brick is placed on the paper.
There are many applications where friction is both useful and necessary. For
example, the brakes on a bicycle create friction between two rubber brake
pads and the rim of the wheel. Friction between the brake pads and the rim
slows the bicycle. Friction is also necessary to make a bicycle go. Without
friction, the bicycle’s tires would not grip the road.
Weather
condition
tires
Rain and snow act like lubricants to separate tires from the road. As a tire rolls
over a wet road, the rubber squeezes the water out of the way so that there can
be good contact between rubber and road surface. Tire treads have grooves
that allow space for water to be channeled away where the tire touches theroad (Figure 5.20). Special irregular groove patterns, along with tiny slits,
have been used on snow tires to increase traction in snow. These tires keep
snow from getting packed into the treads and the design allows the tire to
slightly change shape to grip the uneven surface of a snow-covered road.
Nails Friction is the force that keeps nails in place (Figure 5.21). The material the
nail is hammered into, such as wood, pushes against the nail from all sides.
Each hit of the hammer drives the nail deeper into the wood, increasing the
length of the nail being compressed. The strong compression force creates a
large static friction force and holds the nail in place.
Cleated shoes Shoes are designed to increase the friction between their soles and the ground.
Many types of athletes, including football and soccer players, wear shoes with
cleats that increase friction. Cleats are projections like teeth on the bottom of
the shoe that dig into the ground. Players wearing cleats can exert much
greater forces against the ground to accelerate and to keep from slipping.
5.3 Section Review
1. Explain the causes of sliding friction and static friction.
2. What do you know about the friction force on an object pulled at a constant speed?
3. What factors affect the friction force between two surfaces?
4. Give an example of friction that is useful and one that is not useful. Use examples notmentioned in the book.
torque, rotate, axis of rotation, lineof action, lever arm, rotationalequilibrium
Objectives
3 Explain how torque is created.
3 Calculate the torque on anobject.
3Define rotational equilibrium.
5.4 Torque and Rotational Equilibrium
A canoe is gliding between two docks. On each dock is a personwith a rope attached to either end of the canoe. Both people pull
with equal and opposite force of 100 newtons so that the net force
on the canoe is zero. What happens to the canoe? It is not in
equilibrium even though the net force is zero. The canoe rotates
around its center! The canoe rotates because it is not in rotational
equilibrium even though it is in force equilibrium. In this section
you will learn about torque and rotational equilibrium.
What is torque?
Torque and force Torque is a new action created by forces that are applied off-center to an
object. Torque is what causes objects to rotate or spin. Torque is the
rotational equivalent of force. If force is a push or pull , you should think of
torque as a twist .
The axis of
rotation
The line about which an object turns is its axis of rotation. Some objects
have a fixed axis: a door’s axis is fixed at the hinges. A wheel on a bicycle isfixed at the axle in the center. Other objects do not have a fixed axis. The axis
of rotation of a tumbling gymnast depends on her body position.
The line of action Torque is created whenever the line of action of a force does not pass
through the axis of rotation. The line of action is an imaginary line in the
direction of the force and passing through the point where the force is
applied. If the line of action passes through the axis the torque is zero, no
matter how strong a force is used!
Creating torque A force creates more torque when its line of action is far from an object’s axis
of rotation. Doorknobs are positioned far from the hinges to provide the
greatest amount of torque (Figure 5.22). A force applied to the knob will
easily open a door because the line of action of the force is the width of the
door away from the hinges. The same force applied to the hinge side of the
door does nothing because the line of action passes through the axis of
rotation. The first force creates torque while the second does not.
Reaction torque If you push up on a doorknob, you create a torque that tries to rotate the door
upward instead of around its hinges. Your force does create a torque, but the
hinges stop the door from rotating this way. The hinges exert reaction forces
on the door that create torques in the direction opposite the torque you apply.
This reaction torque is similar to the normal force created when an object
presses down on a surface.
Combining
torques
If more than one torque acts on an object, the torques are combined to
determine the net torque. Calculating net torque is very similar to calculating
net force. If the torques tend to make an object spin in the same direction(clockwise or counterclockwise), they are added together. If the torques tend
to make the object spin in opposite directions, the torques are subtracted
(Figure 5.25).
Calculatingtorque
A force of 50 newtons is applied to a wrench that is 0.30
meters long. Calculate the torque if the force is applied
perpendicular to the wrench at left.
1. Looking for:You are asked for the torque.
2. Given: You are given the force in newtons and the length
of the lever arm in centimeters.
3. Relationships:Use the formula for torque, τ = rF.
4. Solution:τ = (0.30 m)(50 N) = 15 N·m
Your turn...a. You apply a force of 10 newtons to a doorknob that is 0.80 meters away from the edge
of the door on the hinges. If the direction of your force is straight into the door, what
torque do you create? Answer: 8 N·m
b. Calculate the net torque in diagram A (at right). Answer: 10 N·m
c. Calculate the net force and the net torque in diagram B (at right).
An object is in rotational equilibrium when the net torque applied to it is
zero. For example, if an object such as a seesaw is not rotating, you know the
torque on each side is balanced (Figure 5.26). An object in rotational
equilibrium can also be spinning at constant speed, like the blades on a fan.
Using rotational
equilibrium
Rotational equilibrium is often used to determine unknown forces. Any object
that is not moving must have a net torque of zero and a net force of zero.
Balances used in schools and scales used in doctors’ offices use balanced
torques to measure weight. When using a scale, you must slide small masses
away from the axis of rotation until the scale balances. Moving the massincreases its lever arm and its torque. Engineers study balanced torques and
forces when designing bridges and buildings.
5.4 Section Review
1. List two ways in which torque is different from force.
2. In what units is torque measured?
3. Explain how the same force can create different amounts of torque on an object.
4. What is the net torque on an object in rotational equilibrium?
5. A boy and a cat sit on a seesaw as shown in Figure 5.27 . Use the information in the picture to calculate the torque created by the cat.Then calculate the boy’s distance from the center of the seesaw.
Figure 5.26: A seesaw is in rotational equilibrium when the two torques are
17. Why is it much easier to slide a cardboard box when it is empty
compared to when it is full of heavy books?18. Explain two ways friction can be reduced.
19. Is friction something we always want to reduce? Explain.
Section 5.4
20. How are torque and force similar? How are they different?
21. Which two quantities determine the torque on an object?
22. In what units is torque measured? Do these units have the same
meaning as they do when measuring work? Explain.
23. Why is it easier to loosen a bolt with a long-handled wrench than witha short-handled one?
24. In which of the case would a force causethe greatest torque on the shovel? Why?
a. You press straight down on the shovelso it stays straight up and down.
b. You twist the shovel like a
screwdriver.c. You push to the right on the shovel’s
handle so it tilts toward the ground.
25. What does it mean to say an object is inrotational equilibrium?
Solving Problems
Section 5.11. Use a ruler to draw each of the following vectors with a scale of 1
centimeter = 1 newton.
a. (5 N, 0°)
b. (7 N, 45°)
c. (3 N, 90°)
d. (6 N, 30°)
2. Use a ruler to draw each of the following vectors. State the scale you
use for each.
a. (40 N, 0°)
b. (20 N, 60°)
c. (100 N, 75°)
d. (500 N, 90°)
3. Use a scaled drawing to find the components of each of the followingvectors.State the scale you use for each.
a. (5 N, 45°)
b. (8 N, 30°)
c. (8 N, 60°)
d. (100 N, 20°)
Section 5.2 4. Find the net force on each box.
5. A 20-kilogram monkey hangs from a tree limb by both arms. Draw afree-body diagram showing the forces on the monkey. Hint: 20 kg isnot a force!
6. You weigh a bear by making him stand on four scales as shown.Drawa free-body diagram showing all the forces acting on the bear. If hisweight is 1500 newtons, what is the reading on the fourth scale?
7. A spring has a spring constant of 100 N/m. What force does thespring exert on you if you stretch it a distance of 0.5 meter?
8. If you stretch a spring a distance of 3 cm, it exerts a force of 50 N onyour hand. What force will it exert if you stretch it a distance of 6 cm?
Section 5.3
9. Your backpack weighs 50 N. You pull it across a table at a constantspeed by exerting a force of 20 N to the right. Draw a free-bodydiagram showing all four forces on the backpack. State the strength of each.
10. You exert a 50 N force to the right on a 300 N box but it does notmove. Draw a free-body diagram for the box. Label all the forces andstate their strengths.
Section 5.411. You push down on a lever with a force of 30 N at a distance of 2
meters from its fulcrum. What is the torque on the lever?
12. You use a wrench to loosen a bolt. It finally turns when you apply300 N of force at a distance of 0.2 m from the center of the bolt.What torque did you apply?
13. A rusty bolt requires 200 N-m of torque to loosen it. If you can exert amaximum force of 400 N, how long a wrench do you need?
14. Calculate the net torque on the see-saw shown below.
15. You and your little cousinsit on a see-saw. You sit0.5 m from the fulcrum,and your cousin sits 1.5 mfrom the fulcrum. Youweigh 600 N. How muchdoes she weigh?
Applying Your Knowledge
Section 5.1
1. Is it possible to arrange three forces of 100 N, 200 N, and 300 N sothey are in equilibrium? If so, draw a diagram. This is similar to balancing the forces acting on a post.
2. Draw the forces acting on a ladder leaning against a building. Assumeyou are standing half-way up the ladder. Assume the wall of the building and the ground exert only normal forces on the ladder.
Section 5.2
3. Civil engineers analyze forces in equilibrium when designing bridges.Choose a well-known bridge to research. Some of the questions youmight want to answer are listed below.
a. Who designed the bridge?
b. How long did it take to build?
c. Which type of bridge is it?
d. How much weight was it designed to hold?
e. What makes this bridge special?
Section 5.3
4. Many cars today have “antilock breaks” that help prevent them fromskidding. Research to find out how antilock breaks work.
Section 5.4
5. Can an object be in rotational equilibrium but not have a net force of zero exerted on it? Can an object have a net force of zero but not be inrotational equilibrium? Explain your answers using diagrams.